why nRF51822 cannot connect to J-LINK?

[sunnyjet16]

I soldered 5 pieces of PCBA with nRF51822, but I got only one PCBA which can connect to J-LINK. I checked the bad four and found that SWDIO of nRF51822 on the four board is lower than 1V. The right one is 3.3V. My question is:

1. Is SWDIO has to be equal to VDD? Otherwise, the interface does not work.
2. What is the root cause that SWDIO is not equal to VDD? Soldering issue, bad chip,or external circuit? To program the chip, I put only a few circuits on the board including nRF51822 chip, 3.3V LDO which supplys 3.3V to VDD and AVDD.

Regards Alex Chen

[sunnyjet16]

Thanks, Martin. My chip is nRF51822 QFAAH0 I put schematic and layout on the link: pan.baidu.com/.../1mgne2jm

[MartinBL]

It looks like you have a few isolated ground "islands" that is not connected to anything:

Maybe this is the cause? Are the top and bottom ground layers connected? We recommend using a lot of via holes connecting the ground planes. Especially around the antenna.

Are you using the exact same components on all boards?

On the four misbehaving boards are you measuring correct Vdd?